1. Field of the Invention
The present invention relates to a light-emitting device using a light-emitting element which has a film (hereinafter, referred to as an “organic compound layer”) containing an organic compound that emits light through fluorescence and phosphorescence by applying an electric field, and a method of manufacturing the same.
2. Description of the Related Art
A light-emitting element using an organic compound as an illuminant has characteristics such as thinness and light-weight, high response, and driving at a D.C. current and a low voltage. Thus, such light-emitting element is expected to be applied to a next-generation flat panel display. In particular, a display device with light-emitting elements arranged in matrix is considered to have the advantages of a wide viewing angle and excellent visibility, compared with a conventional liquid crystal display device.
According to a light-emitting mechanism of a light-emitting element, by applying a voltage between first and second electrodes with an organic compound layer interposed therebetween, electrons injected from a second electrode and holes injected from a first electrode are recombined at a light-emission center of the organic compound layer to form molecular excitons, and the molecular excitons release energy in returning to a ground state to emit light. As excited states, singlet excitation and triplet excitation are known. It is considered that light can be emitted through either excited state.
Such A light-emitting device with light-emitting elements arranged in matrix can be driven by passive matrix driving (simple matrix type) or active matrix driving, and is capable of conducting a image display. In particular, in the case where the pixel density is increased, active matrix driving in which a switch is provided for each pixel (or one dot) is considered to be advantageous in that driving at a low voltage can be conducted.
An active matrix light-emitting device with a light-emitting element has an element structure as shown in FIG. 2. A thin film transistor (hereinafter, referred to as a “TFT”) 202 is formed on a substrate 201, and an interlayer insulating film 203 is formed on the TFT 202. On the interlayer insulating film 203, a first electrode (anode) 205 electrically connected to the TFT 202 through a wiring 204 is formed. As a material for forming the first electrode 205, a transparent conductive material with a large work function is suitable, and indium tin oxide (ITO) is typically used.
An organic compound layer 206 is formed on the first electrode 205. In the following description, a layer provided between the first and second electrodes will be referred to as an organic compound layer for convenience. The organic compound layer 206 specifically includes a light-emitting layer, a hole injection layer, an electron injection layer, a hole transport layer, an electron transport layer, and the like. Basically, the light-emitting element has a structure in which a first electrode, a light-emitting layer, and a second electrode are stacked in this order. In addition, it may be possible to use a structure in which a first electrode, a hole injection layer, a light-emitting layer, and a second electrode are stacked in this order; a structure in which a first electrode, a hole injection layer, a light-emitting layer, an electron transport layer, and a second electrode are stacked in this order; or the like.
A second electrode 207 is formed after forming the organic compound layer 206, and thus a light-emitting element 209 is formed. As the second electrode 207, metal with a small work function (typically, metal belonging to Group 1 or 2 of the periodic table) is used in most cases.
Furthermore, a first insulating layer 208 of an organic resin material is formed so as to cover an end portion of the first electrode 205 for the purpose of preventing short-circuit between the second electrode 207 and the first electrode 205 at this end portion. In FIG. 2, although a light-emitting element formed in one pixel is shown, actually, a plurality of light-emitting elements are formed in a pixel portion, whereby an active matrix type light-emitting apparatus is formed.
The organic compound layer is formed to have a thickness of about 50 to 150 nm. When the surface of the first electrode (anode) positioned under the organic compound layer is not flat and has minute unevenness, the organic compound layer is not formed uniformly in thickness.
Furthermore, when the insulating surface under the first electrode is not flat and has foreign matters and unevenness with a thickness almost equal to or larger than that of the first electrode, the first electrode is not formed uniformly, and has a surface reflecting the unevenness. Because of this, the organic compound layer is not formed uniformly, either.
As a result, when the second electrode (cathode) is formed on the organic compound layer under the above-mentioned condition, a point (called a dark spot since it is observed as a black point) that does not emit light may be formed in one pixel. Furthermore, a short-circuit is caused vertically between the first and second electrodes.
The organic compound layer is likely to be degraded due to oxygen and moisture. If an organic resin material such as polyimide, polyamide, and acrylic resin is used for the interlayer insulating film and moisture and oxygen occluded in the film are released, a light-emitting element may be degraded due to the gas.
On the other hand, in terms of a TFT, an alkali metal or alkaline-earth metal material, such as Li and Mg, which is a bad impurity of a movable ion with respect to a semiconductor, is used as a material for the second electrode of the light-emitting element. Therefore, in an active matrix type light-emitting device, it is required to prevent the second electrode material from diffusing to a TFT side.